This invention relates to an optical collimator in a data reading device such as a bar code reader, and more particularly to miniaturization of the optical collimator.
A conventional optical collimator, as shown in FIG. 3, comprises: a light source 1 such as a semiconductor laser; a flat glass plate 2; a collimator lens 3; and a holder 4 holding the glass plate 2 and the collimator lens 3.
The light source 1 emits light having an astigmatism and a divergent angle. It is desirable that rays of light outputted by an optical collimator are substantially parallel and have no astigmatic difference. Therefore, it is necessary for the optical collimator to have a function of connecting the astigmatism and converting diverging rays of light into parallel rays of light.
In FIG. 3, in order to correct the astigmatism, the glass plate 2 is held in a inclined position between the light source 1 and the collimator lens 3.
In general, when a flat plate having a thickness of t and a refractive index n is held at an angle of .PHI. in a beam of light having, an, angle of diffusion .theta., then the astigmatism AS can be as follows: EQU As=-(.multidot..phi..sup.2 .multidot.(n.sup.2 -1))/n.sup.3
The astigmatism is moreover directional. Therefore, if the glass plate 2 is positioned so that an astigmatism is provided which is opposite to that of the light source 1; that is, so that the two astigmatisms cancel out each other, then light incident to the collimator lens 3 has no astigmatism, and the rays of light emerging from the collimator lens 3 are substantially parallel, having no astigmatism.
As was described above, the astigmatism can be eliminated by positioning the flat glass plate 2 so that is inclined between the light source 1 and the collimator lens 3. The glass plate 2 thus positioned produces a comatic aberration. The comatic aberration COMA is: EQU COMA=(t.multidot..theta..sup.3 .multidot..PHI..multidot.(n.sup.2 -1))/2n.sup.3
In practice, the comatic aberration obstructs the provision of a uniform optical distribution. Therefore the optical collimator should be free from the coma aberration.
As was described above, the glass plate 2 and the holder 4 are located between the light source 1 and the collimator lens 3, thus requiring a relatively long distance between the light source 1 and the collimator lens 3. Therefore, in order to obtain a predetermined angle of divergence .theta., it is necessary to increase the diameter of the collimator lens 3, and therefore the conventional optical collimator is unavoidably bulky.